Flexible webs of at least slightly elastic material, possibly of combined such materials, are sometimes required to be pulled intermittently through a machine, such as a printing or punching machine, the web being stopped during the operation of the machine. At that time it is often required that the length of stopped web be held under a substantially uniform tension, this applying to each stop in the web's travel during which the machine is operated. For example, a paper web may be pulled through a screen printing machine and then through a punching machine, the web necessarily being stationary during each of these operations, and if the printing machine is to apply a succession of juxtapositioned prints and the following punching requires registration with these prints, for each operation the web must be stopped and held under a substantially uniform tension to insure accurate printing and punching registration.
It is possible to simply stationarily clamp the web pulled into the length to be tensioned and thus hold the web stopped against the pull required to pull the tape onward. However, when the length of web between such a stationary clamp and the length to be tensioned, is large, as when the web must be formed into loops extending from a stationary clamp to the length to be tensioned, difficulties arise. Most webs, particularly paper webs, have some elastic stretchability, and with this characteristic prevailing in the long length between the clamp and the length to be stretched, it becomes substantially impossible to assure uniform tensioning of successive lengths each time the intermittent forward movement of the tape is stopped.
The web is ordinarily pulled by being in friction engagement with an element which, when stopped, anchors the web extending backwardly, and it has been proposed to mount the clamp holding the back end of the web, in a movable fashion so that it can be pulled back a predetermined distance by suitable equipment. This has not worked well because the modulus of elasticity of the web in its various loop sections is usually unpredictable. For example, the modulus of elasticity of paper web and particularly a web of multi-ply material may have varying moduli of elasticity depending on the moisture content of the web.